A Tapered Undulator Experiment At The ELBE Far Infrared Hybrid-Resonator Oscillator Free Electron Laser

A tapered undulator experiment was carried out at the Helmholtz Forschungszentrum Dresden-Rossendorf (ELBE) far-infrared FEL. The oscillator FEL makes use of a hybrid optical resonator. The main motivation was to see whether the presence of a dispersive medium in the form of a waveguide in the resonator has any effect on the outcome. The FEL saturated power and the wavelength shifts have been measured as a function of both positive as well as negative undulator field amplitude tapering. In contrast to the typical high-gain FELs where positive tapering proves beneficial for the output power we observe an improvement of performance at negative taper. During the same experiments we studied the characteristics of the detuning curves. The width of the curves indicate a maximum small-signal gain for zero taper while the output peak power is highest for negative taper. Whereas the saturated power output and the detuning curve characteristics agree with the known theoretical predictions, the wavelength shifts showed deviations from the expected values. Details of the experiment are presented.